Production of cross-linked polyethylene flock

ABSTRACT

Production of cross-linked polyethylene flock by introducing a cross-linking agent and polyethylene into a zone under pressure and under a temperature sufficiently high to support crosslinking of the polyethylene and then abruptly releasing the pressure whereby a cross-linked polyethylene flocculent material is formed.

' United States Patent Inventors Erich Behr Troisdorf; Rolf Beckmann,Seigburg, both of Germany Appl. No. 614,823 Filed Feb. 9, 1967 PatentedNov. 23, 1971 'Assignee Veba-Chemie AG, Gelsenkirchen Buer, GermanyPriority Feb. 14, 1966 Germany D 49359 PRODUCTION OF CROSS-LINKED [56]References Cited UNITED STATES PATENTS 3,452,123 6/1969 Beckmann et al.264/53 2,990,381 6/1961 Meinel 260/25 3,079,370 2/1963 Precopio et al260/949 3,275,577 9/1966 Hoeg et al 260/25 3,098,831 7/1963 Carr 260/253,098,832 7/ I963 Pooley et al. 260/25 3,062,759 I l/l962 Bingham et al.260/25 Primary Examiner-Joseph L. Schofer Assismnl Examiner-Richard A.Gaither Attorney-Burgess, Dinklage & Sprung ABSTRACT: Production ofcross-linked polyethylene flock by introducing a cross-linking agent andpolyethylene into a zone under pressure and under a temperaturesufficiently high to support cross-linking of the polyethylene and thenabruptly releasing the pressure whereby a cross-linked polyethyleneflocculent material is formed.

PRODUCTION OF CROSS-LINKED POLYETHYLENE FLOCK This invention relates tothe cross-linking of polyethylene. It more particularly refers to theproduction of very low-bulk density cross-linked polyethylene.

This specification discloses the peroxide cross-linking of high and lowdensity polyethylene under pressure followed by an abrupt pressurereduction which induces a foamlike expansion in the product to produce aflocculent material.

1t is well known to cross-link polyolefins, and particularlypolyethylene, with peroxides, particularly organic peroxides. It isimportant to provide uniform distribution of the peroxide cross-linkingagent in the polyethylene in order to impart uniform cross-linkingthereto. it is generally conceded that this uniform distribution is bestaccomplished by operating in the fused state, that is by admixing asuitable peroxide with fused polyethylene.

Unfortunately, the melting points of polyethylenes are such that atthese temperatures the usable peroxides have a tendency to break downinto free radicals and thus cause premature cross-linking in anonuniform manner. The premature cross-linking results in a simultaneousincrease in viscosity of the molten polyethylene which causes furtherdifficulty in uniformly distributing the peroxide. In order to overcomethe increased viscosity and attempt to uniformly distribute the peroxidecross-linking agent, increased agitation has been employed but thisgenerally results in increasing the temperature of the melt by frictionthus aggravating the already bad situation and causing scorching,lumping and searing of the melt accompanied by some polymer degradation.Such products are substantially unmarketable as first-class material.

Attempts have been made to rectify this situation by applying thecross-linking agent to a shaped polyethylene article after such has beenformed. This avoids the above set forth problems, but also results in anonuniforrnly cross-linked product.

Foam-form polyethylene is a desirable product which has been produced bythe use of suitable blowing agents in combination with fused polymer.Although it has been possible to cross-link such foam after forming,such product is subject to the same nonuniformity as has been discussedabove. Attempts to cross-link and foam simultaneously through the use ofblowing and cross-linking agents simultaneously admixed with moltenpolymers have not been successful. One of the problems in providing sucha cross-linked foam is that the blowing agent must have a decompositiontemperature which is higher than the melting point of the polymer butlower than the decomposition temperature of the cross-linking peroxide.

It is therefor an object of this invention to provide an improvedprocess for the production of low-bulk density crosslinked polyethylene.

lt is another object of this invention to provide a novel process forproducing cross-linked polyethylene.

Other and additional objects of this invention will become apparent froma consideration of this entire specification including the claimsappended thereto.

In accord with and fulfilling these objects, one aspect of thisinvention resides in a process comprising melting polyethylene in apressuri'zable mill or kneader; adding peroxide cross-linking agent;working the mass until cross-linking is complete; and abruptly releasingthe pressure. Upon release of the pressure, a finely flocculent, whiteproduct is produced.

The pressure required for the process of this invention is preferablyprovided by filling the mill or kneader to at least full capacity withpolyethylene such that closing the cover of the mill or kneader impartsa pressure upon the polyethylene. The progress of the cross-linkingprocess can be followed by measuring the increase in viscosity of themelt since as the degree of cross-linking increases so does theviscosity of the polyethylene melt. As the viscosity of the meltincreases, greater kneading power is required thus drawing largeramperage which is itself a measure of the degree of cross-linking. Sinceit has been found that the cross-linking process is accelerated byincreasing the kneading speed, it is preferred to increase this speed ascross-linking proceeds thus requiring still further increase in powerinput to the kneader. 1t is also preferred to raise the temperature ofthe melt after mixture of the peroxide therewith in order to acceleratecross-linking.

The product of the process of this invention may be dried in order toremove any of the peroxide decomposition products therefrom which may beundesirable.

In contrast to the starting polyethylene, the product is insoluble inall known solvents. The melting range of the flocks is usually 50 to C.higher than the melting range of the starting product, depending on thetype of polyethylene used. The flocks can also be compressed at lowtemperatures under high pressure to form porous boards, or to bond themto solid polyethylene boards or other articles.

The flocks possess a bulk weight of 0.2 to 0.3 gJcm.

All known polyethylenes and ethylene copolymers having densities of 0.92to 0.965 are suitable for this process.

The peroxides commonly used for the cross-linking of polyethylenes areused as the peroxides in this process, that is, mainly dialkyl peroxidesor, for example, dicumyl peroxide. The amount of peroxide requireddepends to some extent on the polymer that is to be cross-linked.Low-density polyethylene runs between 2.5 and 6 wt. percent.High-density polyethylene runs between 3.5 and 6 wt. percent. Thegreater the amount of peroxide that is used, the finer and more uniformthe flocks of the resulting product willbe.

The melting temperature runs around 1 10 to C. for a low-densitypolyethylene, and around to C. for a high-density polyethylene. It isbest to operate on the two-current principle, i.e., one thermostat isset to the plastification temperature, and another to the cross-linkingtemperature to and the temperature control of the kneader is switched tothe second thermostat after the plastification and the working in of theperoxide. The process is preferably carried out at elevated pressures.

The thermal and chemical stability of the product makes it suitable forvarious insulating purposes, as for example, for

hot water insulation, and also for insulation against cold and noise. Onaccount of its excellent electrical properties, the product can be usedfor various purposes in the electrical industry, as for example, for useas a dielectric separating layer in cable insulation. Also, compressedand densified flocks can be used as liquid or gas filters having specialresistance to chemicals. Molded insulators can also be made by pressing.

This invention is illustrated by the following example which is in noway limiting thereon.

EXAMPLE TABLE Bulk Weight of the Flocks using Amount of PolyethylenePolyethylene Peroxide Density 0.918 Density 0.95 5 (1: by weight) ing./cm.. in g./cm.-".

polymer is polyethylene having a density of about 0.92 to 0.965 gram percubic centimeter.

3. The improved process claimed in claim 1, wherein said peroxide isdicumyl peroxide.

4. The improved process claimed in claim 1, wherein said peroxide ispresent in proportion of about 2.5 to 6 weight percent.

5. The improved process claimed in claim 1, carried out at about l50 tol C.

6. The improved process claimed in claim 1. wherein the product bulkdensity is about 0.2 to 0.3 gram per cubic centimeter.

2. The improved process claimed in claim 1, wherein said polymer ispolyethylene having a density of about 0.92 to 0.965 gram per cubiccentimeter.
 3. The improved process claimed in claim 1, wherein saidperoxide is dicumyl peroxide.
 4. The improved process claimed in claim1, wherein said peroxide is present in proportion of about 2.5 to 6weight percent.
 5. The improved process claimed in claim 1, carried outat about 150* to 190* C.
 6. The improved process claimed in claim 1,wherein the product bulk density is about 0.2 to 0.3 gram per cubiccentimeter.